The cam contacting face of tappets used in internal combustion engines is subjected to the significant abrading and heat inducing action of a cam repeatedly striking and rubbing across it. As this face becomes worn, tappet travel is changed causing valve motion to be altered consequently affecting engine performance. Eventually, engine performance is decreased to the point where rectification of this problem is mandatory. Wear of the tappet can also result in damage to the cam that drives it.
Extensive efforts have been made to minimize tappet wear so as to avoid the consequent problems. Most attempts at a solution to this wear have centered on providing a hardened face for cam contact. In U.S. Pat. No. 2,817,144 issued to Zeller on May 25, 1953 there is disclosed a tappet with a carbide facing attached thereto by brazing. This development and refinements thereof are commonly used improvements on the standard tappet. All suffer the defect of relying on a brazed joint for adherence of the wear resisting face to the tappet proper. Stresses on this joint due to differing coefficients of thermal expansion between the joined surfaces can result in separation of the wear resisting surface from the tappet.
Many other schemes for joining hardened surfaces to tappets have been proposed, none of which yield a totally satisfactory product. Robinson et al in U.S. Pat. No. 3,198,182 issued Aug. 3, 1965 describe a tappet including a hardened wear face formed and bonded to the tappet by techniques of powder-metallurgy. Such tappets are very expensive and consequently are not widely used.
German Pat. No. 2,209,926 issued to Bertinetti on Sept. 14, 1972 discloses a tappet formed of a steel wear face implanted in a plastic or nylon main body. In this invention, the wear face is embedded in the tappet during the molding of the plastic main body. Such a tappet would be limited in application to engines in which the tappet temperature and mechanical loadings are sufficiently low so as to be within the material limitations of the synthetic substance from which the main body is fabricated.
French Pat. No. 1,020,632, issued to Robig on Feb. 9, 1953, relates to a tappet with a hardened cam contacting wear face made of cast iron or some ceramic material. In this invention the insert is retained in place by a dovetailed joint, the tappet main body being cast with the prefashioned dovetailed wear surface in place. A drawback of this invention is the expense of casting the tappet with the wear resisting insert in place. Also, if the insert is metallic, detrimental metallurgic changes can occur within it should its temperature be sufficiently elevated in the process of casting the tappet main body around it.
The present invention is directed to overcoming one or more of the problems as set forth above.